A crystalline polypropylene has been extensively used in the field of various moldings, because of good mechanical properties, good chemical resistance or the like, very useful balance with economy. However, the use of a propylene homopolymer as the crystalline polypropylene brings about an increase in rigidity, but lack of impact resistance. Therefore, an improvement in impact resistance has been made by a method of adding an ethylene/propylene rubber, an ethylene/propylene/diene rubber or the like to the propylene homopolymer and a method for the polymerization of propylene alone, followed by the copolymerization of ethylene and propylene to produce a so-called block copolymer. However, more improvement in balance of rigidity and impact resistance has been desired. Further, there is a strong desire for the improvement in performance of polypropylene, in particular its heat resistance and toughness with the aim of expanding the use limited in prior polypropylene.
It is known that an isotactic polypropylene is produced by the polymerization of propylene, using a catalyst consisiting of a combination of a metallocene and an aluminoxane which is different from the prior catalyst systems. It is also known that the so-called block copolymer is produced by the polymerizatiuon of propylene alone using a similar catalyst, followed by the copolymerization of ethylene and propylene.
For example, Japanese Patent Kokai Hei 4-337308 discloses an example using as the metallocene dimethylsilyl bis(2,4-dimethylcyclopentadienyl)zirconium dichloride, Japanese Patent Kokai Hei 6-287257 discloses an example using dimethylsilyl bis(methylcyclopentadienyl)hafnium dichloride, Japanese Patent Kokai Hei 5-202152 and Hei 6-206921 disclose an example using rac-ethylene bis(indenyl)hafnium dichloride, rac-dimethylsilyl bis(indenyl)hafnium dichloride and rac-phenylmethylsilyl bis(indenyl)hafnium dichloride and Japanese Patent Kokai Hei 6-172414 discloses an example using dimethylsilylene bis(2-methylindenyl)zirconium dichloride and dimethylsilylene bis(tetrahydroindenyl)zirconium dichloride.
The propylene/ethylene block copolymers produced by use of those metallocenes have achieved to some extent an improvement in impact resistance, but there is room for improvement in rigidity and heat resistance.
Thus, a problem involved in the polypropylene composition produced by known processes is more improvement in the balance of rigidity and impact resistance, heat resistance and toughness.